This description relates to a light emitting device and a method for fabricating the same. Typically, a light emitting device is a light source having one wavelength for various applications in, for example, a light source and a display.
Most of the lights generated from inside of the light emitting device are trapped within the light emitting device by reflection from an interface between the two materials such as a semiconductor and air beyond a critical angle. In other words, light that reaches the surface beyond the critical angle will not cross but will experience a total internal reflection (TIR).
Referring to FIG. 1 which is a conceptual drawing illustrating a light path formed by a difference of index of refraction between two materials according to prior art.
According to Snell's law of Equation 1, when a light is directed from one material with an index of refraction n1 towards another material with an index of refraction n2, the light will be refracted if its incident angle is smaller than a critical angle. Otherwise, the light will be totally reflected from an interface between the two materials beyond a critical angle.nl*sin θ1=n2*sin θ2  Equation 1where, θ1 is an incident angle while θ2 is a refraction angle.
FIG. 2 is a schematic cross-sectional view illustrating a light path in a light emitting device according to the prior art. The light emitting device sequentially from bottom to top includes a substrate (10), an N-semiconductor layer (11), an active layer (12) and a P-semiconductor layer (13), where lights (a,b,c) traveling to the outside of the device at an angle less than a critical angle out of lights emitted from the active layer (12) will cross, while light (d) that reaches the outside of the device beyond the critical angle will not cross but will be trapped inside the device and experience total internal reflection (TIR).
As a result, if quantum being trapped in the light emitting device increases, an output of light from the light emitting device decreases, degrading its efficiency. There are several approaches for improving light extraction efficiency from the light emitting device.
In one approach to improving the light extraction efficiency, light emitting devices are ground into hemispherical shapes. In other words, a light emitting surface of the light emitting device is shaped into a hemisphere with an emitting layer at the center. Light emitted from a point in the active region of a hemispherically shaped light emitting device intersects the hemispherical interface at near normal incidence. Thus, total internal reflection is reduced. However, this technique, although it is one of the best optical choices, is tedious and wasteful of material. In addition, defects introduced during the grinding process may compromise the reliability and performance of the light emitting devices.
In another approach, light emitting devices are encapsulated (encased) in a material with a dome or hemispherically shaped surface, although it is very difficult to manufacture. In still another approach, a substrate re-absorbing light emitted from the light emitting devices is changed to a substrate for total reflection.
In a different approach, a light emitting device having a micro cavity structure or a resonant cavity structure is disclosed. But this approach requires a very precise controllability and reproducibility relative to thickness of the structural layers during the fabrication process, and if the light is to be effectively extracted from semiconductors to air, this approach has a shortcoming in that the emitting wavelengths of the light emitting device should accurately match the fabricated cavity mode. Another shortcoming is that the emitting wavelengths of the light emitting device vary to drastically reduce the light extraction if temperatures or operating currents increase.
Recently, as a means to reduce TIR and improve overall light extraction, one of the more popular approach is developed which is a surface texturing. The surface texturing technique is to roughen a surface of a light emitting device chip from which light generated thereinside is artificially emitted or to include a periodic pattern of the emitting surface.
The approach of the surface texturing technique known to enhance the light extracting efficiency from the light emitting device chip can be used individually, or can be applied in association with the known techniques such as the chip shape-changed technique, the epoxy encapsulation and the substrate change approach. The surface texturing approach has been shown to improve light emission efficiency to a great extent.
The current surface texturing is patterned into the light emitting device surface as a mask during a dry or wet etching. One shortcoming of the surface texturing is that height of the surface shape is limited due to a predetermined thickness of each structural layer, and a very accurate controllability and reproducibility relative to thickness of the structural layers during the etching process, is required. Another shortcoming is that many processes including pattern forming and the like for etching are required.
There has been recently a great deal of heightened interest and development in light emitting devices of Group-III nitride based material systems having wide band gaps by which nitride based semiconductor growth structure or grown epitaxial fabricating process is improved to thereby enhance the photo conversion efficiency.
For example, light emitting devices employing semiconductor nano structures such as nanorod and nanowire can alleviate stress over the conventional thin-film semiconductors to enable to enhance the internal quantum efficiency and to structurally improve the light extraction efficiency.
The method for forming a nano structure can be categorized into two types, that is, a process for growing a nano structure, and a process for realizing a nano structure by patterning the conventional thin-film epitaxial wafer and etching the same.
Many techniques are developed and performed by research institutes for using metal as catalyst or growing GaN nanorod and nanowire without using catalyst. However, there is a shortcoming in the growing process thus explained in that it is difficult to grow an epitaxial layer of a device having a nano structure and it is difficult to have a reproducible nano structure shape having a predetermined length and thickness.
Another shortcoming is that a nano structure manufacturing method using semiconductor processing technology (e.g., photolithography technology) is, however, associated with the problems of poor manufacture yield and high system cost, although light extraction efficiency has been improved and the internal efficiency has been enhanced through the alleviation of stress by manufacturing of micro LED and nanorod LED by etching a sample with a patterned surface via the photolithography technology.